{"id":7721,"date":"2012-09-17T15:45:57","date_gmt":"2012-09-17T14:45:57","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=7721"},"modified":"2012-09-17T15:45:57","modified_gmt":"2012-09-17T14:45:57","slug":"predicted-properties-of-a-candidate-for-a-frozen-semibullvalene-2","status":"publish","type":"post","link":"https:\/\/rzepa.net\/blog\/2012\/09\/17\/predicted-properties-of-a-candidate-for-a-frozen-semibullvalene-2\/","title":{"rendered":"Predicted properties of a candidate for a frozen semibullvalene."},"content":{"rendered":"

I am following up on one unfinished thread in my previous post<\/a>; a candidate was proposed in which the transition state for [3,3] sigmatropic rearrangement in a semibullvalene might be frozen out to become instead a stable minimum.<\/p>\n

\"\"<\/p>\n

The hypothesis was that such a species would be aromatic, a bis<\/em>-homoaromatic species to be precise, itself an interesting variation on the normal theme of aromaticity. The so-called computed [3,3] vibrational mode emerged as real and not imaginary. But how to go about showing that the resulting species might actually be aromatic? I started with a QTAIM characterisation of bond and ring critical points.<\/p>\n

\"\"
QTAIM analysis of the proposed frozen semibullvalene. Bond critical points in magenta. Click for 3D.<\/figcaption><\/figure>\n

The two homobonds are shown above with dashed bonds. In fact, there are no bond critical points (bcp) located between these two atoms. This of itself need not distress us; the lack of a suitable bcp has been observed previously for other such homo-aromatics[cite]10.1021\/jo801022b[\/cite], a phenomenon referred to as a no-bond bond<\/em>. But because the QTAIM does not locate a bond, there is no ring located for the aromatic system either. And in such a heavily non-planar system, the lack of an unambiguous ring centroid means that it is not clear where to place any NICS probe of aromaticity.<\/p>\n

What about the ELF technique. For M\u00f6bius homoaromaticity[cite]10.1021\/ct8001915[\/cite] the homo-bond does display a locatable disynaptic ELF basin integrating to around 1-electron. No so here! So no progress yet on estimating the aromaticity.<\/p>\n

\"\"
ELF analysis for the frozen [3,3] system. Disynaptic ELF basin centroids in magenta. Click for 3D.<\/figcaption><\/figure>\n

Are there any other computable properties which might be detected experimentally? Well, I noted that the species is C2<\/sub>-symmetric and hence chiral. What about its computed electronic circular dichroism spectrum? These are shown for firstly<\/a> the frozen system, for which both C-N bonds are 2.13\u00c5 in length, and then for a distorted version<\/a> in which one C-N bond is 1.6\u00c5 and the other 2.4\u00c5. The former is more delocalised (as expected for an aromatic ring) and so has\u00a0\u03bbmax<\/sub> in a region which would give the compound a mauve colour. The latter is less delocalised and hence would have an orange-yellow colour, with \u03bbmax <\/sub>some 100nm less. The ECD spectra also have different appearances. This would be indeed an interesting way of determining changes in delocalisation, in this case possibly associated with homo-aromaticity.\u00a0<\/p>\n

\"\"<\/p>\n

\"\"<\/p>\n","protected":false},"excerpt":{"rendered":"

I am following up on one unfinished thread in my previous post; a candidate was proposed in which the transition state for [3,3] sigmatropic rearrangement in a semibullvalene might be frozen out to become instead a stable minimum. The hypothesis was that such a species would be aromatic, a bis-homoaromatic species to be precise, itself […]<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[11],"tags":[1884],"class_list":["post-7721","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry","tag-pericyclic"],"_links":{"self":[{"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/posts\/7721","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/comments?post=7721"}],"version-history":[{"count":0,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/posts\/7721\/revisions"}],"wp:attachment":[{"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/media?parent=7721"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/categories?post=7721"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/tags?post=7721"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}